Polymer particles having a particle size of 1 to 100 μm are utilized for various uses, such as for an additive (e.g. a flatting agent) for a coating agent such as a coating material, an additive (e.g. a flatting agent) for ink, a main component or an additive for an adhesive, an additive (e.g. a shrinkage reducing agent) for cultured marble, a paper treating agent, a filler (a filler for improving lubricity) for an external preparation such as cosmetics, a column packing material for chromatography, a toner additive for developing an electrostatic image, an anti-blocking agent for a film, and a light diffusing agent for a light diffuser (e.g. a light diffusion film).
The above polymer particles are generally produced by suspension polymerization of a polymerizable monomer. In the process of suspension polymerization, droplets of a polymerizable monomer are dispersed in an aqueous medium in the presence of a suspension stabilizer (a dispersant) by a mechanical shear force with use of a stirrer, and polymerization proceeds within the droplets of the suspended polymerizable monomer. Since suspension polymerization proceeds in a state where the droplets are dispersed, the distribution and the size of the droplets depend on the characteristics, structure, shape, rotational speed, etc. of the stirrer. However, even if the size of the droplets can be miniaturized, droplets may coalesce or flocculate with each other during polymerization, unless the dispersion stability of the droplets can be maintained. Hence, it is important in suspension polymerization to stabilize the droplets of the polymerizable monomer after the droplets are finely dispersed, and thereby to prevent coalescence of the droplets.
In order to prevent coalescence or flocculation of droplets during the polymerization process without fail, choice of the suspension stabilizer is important. Use of various suspension stabilizers have been reported in relation to suspension polymerization. Commonly employed suspension stabilizers include, for example, water-soluble polymers (such as polyvinyl alcohol, gelatin, and starch) and poorly soluble, finely powdered inorganic compounds (such as poorly water-soluble salts including barium sulfate, calcium carbonate, calcium hydroxide, aluminum hydroxide, tricalcium phosphate, and magnesium pyrophosphate; and powders of silicic acid, clay, silica, and metal oxides).
Nevertheless, there are some disadvantages in using the above water-soluble polymers as the suspension stabilizer. First, the particle size distribution of the particles obtained by suspension polymerization is undesirably wide. Second, emulsion polymerization which coincides with suspension polymerization generates many fine particles (particles whose particle sizes are finer than the desired particle size range). Third, it is difficult to remove water-soluble polymers which adhere to surfaces of polymer particles.
On the other hand, the particle size distribution can be comparatively narrowed if the above inorganic compounds are employed as the suspension stabilizer. For example, PTL 1 discloses polymer particles (resin particles) obtained by suspension polymerization of a polymerizable monomer in an aqueous medium, using silica as a suspension stabilizer (a dispersant). In the resulting polymer particles, the coefficient of variation (CV) in particle size is between 10% and 50%.